1. Field of the Invention
The present invention relates to a photocoupling device which can be used in optical data communication, and in particular, to a photo-coupling device capable of accomplishing functions such as light wave switching and dividing, or branching by controlling the coupling condition between a pair of optical (light) wave guides.
In optical data communication, technologies are known for transmitting light waves in optical data media, and optical elements for processing light waves according to various purposes, such as branching, coupling, and filtering. While optical fiber is generally used for the longdistance light wave transmission, an optical film waveguide is used as a local light wave transmitting device. The optical film waveguide is designed such that a strip having a slightly larger refractive index is formed over the surface of a dielectric plate, and light waves are transmitted as enclosed therein. In this case, if a substrate of electro-optical crystal material is used as a dielectric plate, an optical element having various functions can be made, since the refractive index would vary according to changes of the applied voltage. The present invention relates to improvements of the photo-coupling device as one of the optical elements employing said electro-optical substrate.
2. Description of the Prior Art
FIG. 1 shows an example of the photo-coupling device employing a prior art electro-optical substrate. In this photo-coupling device, a pair of optical waveguides W1 and W2 are formed on the electro-optical substrate 1, and the coupling condition between the optical waveguides W1 and W2 is varied by changing the refractive indices of the optical waveguides by applying a voltage between plate electrodes 2 (2a, 2b) and 3 (3a, 3b) arranged above the optical waveguides, respectively. As evident from a graph shown in FIG. 2, coupling is intensified at a point where the effective refractive indices of both optical waveguides (shown by solid lines) become equal for a certain applied voltage Vo, thereby allowing to transfer the light wave of the center wavelength .lambda.o from one waveguide to the other. As the applied voltage is changed to V1, the effective refractive indices of said two optical wave guides PG,5 become as shown by the broken lines, and the coupling for the light wave of center wavelength .lambda.1 at the merging point of these two broken lines is enhanced, thereby causing the light wave to transfer from one optical waveguide to the other.
However, the prior art photo-coupling device shown in FIG. 1 is very poor in terms of wavelength selectivity with the coupling width being in the order of about 200 .ANG., and, consequently, it suffers from a disadvantage that the extent of waveform multiplexing is limited, particularly for the wavelength multiplexed optical communication. Another disadvantage is that when the impressed voltage is changed, the points at which the effective refractive indices of the two optical waveguides coincide are not continuous, and therefore transferring of the light wave by continuously changing the center wavelength (frequency ) cannot be made.